1. Field of the Invention
The present invention generally relates to an apparatus for controlling and providing power to an electrodeless fluorescent lamp, and in particular to a fluorescent light ballast that will dim an unmodified electrodeless lamp across a very wide dimming range.
2. Description of Related Art
Liquid crystal materials emit no light of their own. They do however reflect and transmit light from external light sources. Liquid Crystal Displays (xe2x80x9cLCDxe2x80x9d) are rapidly becoming the display material of choice for new, high-technology displays.
Accordingly, it is necessary to back light the LCD, which is typically done with fluorescent lamps located directly behind the LCD. When illuminated from behind, a white diffusion sheet between the backlight and the LCD redirects and scatters the light evenly to ensure a uniform display.
Fluorescent lights have a limited useful life and become dim over time. However, there is a new type of electrodeless fluorescent lamp available today that has a substantially longer life span than the electrode variety. Such lamps are manufactured and sold by Sylvania under the trade name ICETRON. These lamps are oval in shape and include a coil at each end for excitation thereof
A method was developed for dimming such electrodeless lamps using a standard ballast. An additional winding was added to one core of the lamp. This prior art method operated by pulse width modulating a shorting switch across the winding of the drive coil. However, this prior art method has a limited dimming range of less than 100:1 and has poor efficiency.
Therefore, a need exists for an optimum dimming control for a fluorescent lamp operating in back lighting displays requiring up to a 10,000:1 brightness range.
Accordingly, a feature of the present invention is the provision of high intensity light source for use as a back light for LCD""S.
Another feature of the present invention is the provision of a dimmable light source operating with a 400 volt square wave at a resonant load frequency of 230 KHz.
Yet another feature of the present invention is the provision of a light source ballast that will switch the drive voltage between a normal operating frequency selected for maximum power transfer (i.e., maximum brightness) of the electrodeless lamp and a higher frequency selected for minimum power transfer (i.e., for black-out, or turn off condition). For a lamp designed to operate at 250 KHz, a preferred normal operating frequency is 260 KHz and a preferred non-operating frequency is 360 KHz.
Still another feature of the present invention is the use of pulse width modulation for changing the brightness of an electrodeless lamp by varying the time that a drive voltage is supplied at either of two distinctly different frequencies: one frequency turning the lamp on at maximum brightness and the other frequency turning the lamp completely off. A variety of digital and analog pulse width modulation (PWM) sources are readily available to provide a dimming control input. A feature of the present invention is the use of a synchronous digital dimming control.
These and other features, which will become apparent as the invention is described in detail below, are provided by an apparatus for controlling the brightness of an electrodeless fluorescent lamp excited by power supplied to a coil disposed in closed proximity to said lamp. The apparatus includes a dimming control unit responsive to a brightness setting for the lamp that provides a PWM signal indicative of the brightness setting. An oscillator is disposed for producing a first lamp drive voltage having a first frequency selected in response to a first state of the PWM signal and a second lamp drive voltage having a second frequency selected in response to a second state of the PWM signal. Accordingly, the first drive voltage turns on the lamp by transferring maximum power to the lamp and the second drive voltage turns off the lamp by transferring minimum power to the lamp.
Still other features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein is shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive, and what is intended to be protected by Letters Patent is set forth in the appended claims. The present invention will become apparent when taken in conjunction with the following description and attached drawings, wherein like characters indicate like parts, and which drawings form a part of this application.